1. Field of the Invention
The present invention relates to image forming apparatuses such as a copying machine and a printer in which an electrophotographic system, and an electrostatic recording system are used.
2. Related Background Art
In a conventional image forming apparatus, in a transfer process, by using a corona charger or another transfer charging means to apply an electric charge having a polarity reverse to that of toner to the reverse surface (a surface opposite a surface abutting on a photosensitive body) of a transfer sheet as a recording medium abutting on a photosensitive body which is a latent image bearer bearing a toner image, the toner image is electrostatically transferred onto the transfer sheet. The transfer sheet adheres to the photosensitive body in this process. To solve the problem, in the image forming apparatus, subsequent to the transfer process, by using a corona charger for eliminating electricity to apply an electric charge having a polarity reverse to that of the transfer process to the reverse surface of the transfer sheet, the transfer charge is eliminated to separate the transfer sheet from the photosensitive body.
The separatability of the transfer sheet from the photosensitive body changes with the type and storage state of the transfer sheet for use. Particularly, among recycled papers frequently used in recent years, there are papers whose paper fibers are softened in a manufacture process and whose rigidities are low. When such recycled paper having adhered along the curved surface of the photosensitive body in the transfer process tries to be separated, it cannot easily be separated because of a low resiliency to a bending deformation, which causes a poor separatability.
Moreover, if the sheets are stored under a humid environment largely different from a manufacture environment or in a non-flat place, they tend to be curled before use. When the transfer sheet is kept being curled in a winding direction to the curved surface of the photosensitive body and is supplied, the poor separatability is caused. The transfer sheet having failed in separation contacts other constituting components disposed downstream of the separation position and causes paper jamming.
To solve the problem, a conventional art comprises curl applying means in the upstream position of a transfer sheet conveying direction to a transfer position midway in a transfer sheet conveyance path, and the supplied transfer sheet prior to a transfer processing is curled by the means in a direction reverse to the winding direction to the photosensitive body, so that the separatability is preserved.
In the conventional image forming apparatus, however, when the transfer sheet is curled as described above, the curl causes a transfer failure in the rear end portion of the transfer sheet in some cases.
The reason why such transfer failure occurs will be described with reference: to FIGS. 11A and 11B.
In FIGS. 11A and 11B, FIG. 11A shows that the toner image is transferred to an area other than the rear end portion of the transfer sheet, and FIG. 11B shows the transfer to the rear end portion. Here, an example is shown in which the toner charging polarity is plus and the transfer current is minus (a minus transfer charge is applied to the transfer sheet).
In FIG. 11A, a transfer corona charger 5 as transfer charging means is connected to a high voltage power source 102, and a transfer electric charge 5c is discharged from a discharge electrode 5a. Thereby, the transfer charge 5c is applied to the back surface (on the side of the transfer corona charger 5) of a transfer sheet P guided by a guide member 9 and conveyed to a transfer position 5b, and toner image T0 on a photosensitive body 1 as a latent image bearer is electrostatically transferred onto the transfer sheet P. In this process the transfer sheet P is held to contact the photosensitive body 1, and all the toner images are transferred onto the transfer sheet P.
On the other hand, in FIG. 11B, since a rear end portion Q is deformed in a direction apart from the photosensitive body 1 by an influence of curl application, a part of the transfer charge passes between the rear end portion of the transfer sheet P and the photosensitive body 1 and turns to the top surface (on the side of the photosensitive body 1) of the transfer sheet P to minus-charge the toner image. Therefore, a minus charged toner image T1 is subjected to resiliency from the transfer charge of the back surface of the transfer sheet P, and returns onto the photosensitive body 1. Therefore, a part of toner is not transferred in the rear end portion of the transfer sheet P, and a transfer null area is generated. Particularly under the low humidity environment, since the transfer sheet contains little moisture and has a high surface resistance, the transfer charge does not easily flow on the back surface of the transfer sheet P and easily flows to the photosensitive body 1, thereby easily generating the transfer null area.
An object of the present invention is to provide an image forming apparatus which applies a curl to a transfer material to prevent the transfer material from winding around an image bearer.
Another object of the present invention is to provide an image forming apparatus which prevents a transfer null area from being generated in the rear end of a transfer material.
Further object of the present invention is to provide an image forming apparatus which comprises:
an image bearer for bearing a toner image;
transfer means for electrostatically transferring the toner image on the image bearer to a transfer material; and
control means for controlling a transfer current of the transfer means.
The control means reduces the transfer current to the rear end portion of the transfer material.
Further objects of the present invention will be apparent from the following description.